János Bakai’s scientific contributions

An advantage of acoustic emission (AE) monitoring is that it can be used to capture pre-peak, peak, and post-peak stress changes in the rock mass. It provides useful information for characterizing rock mass behaviour at all stages of fracturing, even prior to measurable deformation. Based on the fact that AE activity is closely related to stress change in the rock mass and the stress level for AE initiation is associated with the uniaxial compressive strength of the rock mass, one can utilize this understanding of brittle failure to determine the relative stress state in the rock mass. An objective function is derived from the consideration of stress in the rock mass and rock mass strength using the generalized AE initiation threshold. Th e seismo-acoustic sensors used in the Bátaapáti National Radioactive Waste Repository (NRWR) contains an acceleration sensor of type PI-A-3-1, which is a self-developed system by Hungarian specialists (Geopolita Ltd.), enabling acceleration measurement. High-bandwidth, high-sensitivity piezoelectric acceleration sensor signals are received by lownoise preamplifi ers. Aft er amplifi cation and fi ltering, the X, Y, Z direction acceleration signals are received by a three-channel analog summing-converter. Th e summed signals are sent to the central unit via a shielded cable aft er the undercut fi lter. Th is paper is presenting the theoretical background of AE measurements used in Bátaapáti NRWR and a detailed backanalysis study on the measured database which provides the description of the stress-strain behaviour of rock mass and the prediction of the short- and long-term rock mass strength. Most back-analysis in geotechnical engineering is based on methods that utilize fi eld displacement monitoring data. In this paper, the investigation of the rock mass strength is examined using a novel method which was developed to backcalculate rock mass strength parameters from acoustic emission (AE) monitoring data in combination with fi nite element method (FEM) and boundary element method (BEM) stress analysis. Th e rock mass strength parameters identifi ed from this approach compare well with the strength data predicted from the geotechnical fi eld mapping and fi eld test data, suggesting that the procedure can be used eff ectively to back-calculate rock mass strength parameters from AE monitoring data. Displacement measurements like extensometers and convergence meters are routinely conducted in situ measured movements. Displacement is one of the reliably measurable quantities in the fi eld. Since deformations are also event-sequence and/or time-dependent and they are the most readily available fi eld measurement for any kind of calibration, they can be used to back-analyze specifi c or abnormal deformation events. However, large abnormal deformations are usually associated with the failure of the rock mass in the post-peak region. Oft en, deformation control measures around the residual strength level are not necessarily eff ective in preventing large, abnormal deformations, so the use of AE monitoring is crucial.

In order to understand how the in-situ primary stress state has evolved with subsidence and uplift in a granitic rock mass for anticipated of a radioactive waste repository in Hungary, the authors investigated the applicability of seismic tomography as an interpretive tool. Very high P wave velocity (V p) values were obtained during the tomographic scanning of the study area of the repository, and these were compared with existing findings of in-situ and laboratory seismic measurements. Apart from seismic tomographic survey, dynamic FEM numerical modelling, empirical calculations of residual stresses, laboratory measurements of compression wave (ultrasonic) velocities on intact rock cores, in-situ primary stress measurements as well as site geological model were integrated to evaluate the use of seismic tomography for identifying possible in-situ stress increases around the excavation. A detailed calibration modelling was carried out based on the site seismic tomography measurements and during the large-scale modelling. It was observed that the increasing V p is directly related to simulated increasing directional loadings on the rock mass. Using a measured wave raypath it was possible to check the different in-situ stress parametrizations which resulted in the best approximation to the measured V p values. It was concluded that the rock mass under investigation to extend the repository must have higher in-situ stress values than the area of the constructed deposition chambers nearby. The results of this research indicated that seismic tomography is a useful tool for determining relative stress around and within the vicinity of underground excavation.

ABSTRACT: The program for the final disposal of low and intermediate level radioactive waste wasestablished by Paks Nuclear Power Plant, Hungary. The Central Nuclear Financial Found and the PublicLimited Company for Radioactive Waste Management (PURAM) has been established to coordinateorganizations and activities for all tasks in connection with nuclear waste management. Based on a detailedgeological screening a granite complex has been chosen as potential host rock beside Bátaapáti village inthe south-western part of Hungary. The goal of this paper is to present shortly the seismoacoustic systemsused in underground facilities and to give and discuss more detailed information on the long term acousticemission (AE) acquisition system installed in the repository area. The used AE measurement methodallowed a detailed short- and long-term, non-destructive supervision. With this technique is possible toallocate zones of stress-related fracturing around the underground spaces and to correlate the modelingresults of the Excavation Damaged Zones (EDZs) around the Bátaapáti radioactive waste repository.